U.S. patent number 7,619,042 [Application Number 11/972,768] was granted by the patent office on 2009-11-17 for polyimide polymer with oligomeric silsesquioxane.
This patent grant is currently assigned to NeXolve Corporation. Invention is credited to Brandon Farmer, Garrett Poe.
United States Patent |
7,619,042 |
Poe , et al. |
November 17, 2009 |
Polyimide polymer with oligomeric silsesquioxane
Abstract
A soluble polyimide polymer with tethered oligomeric
silsesquioxane compounds is produced using efficient, gentle
reactions. A carboxylic acid attachment point on the polymer
backbone is used to connect the oligomeric silsesquioxane. The
oligomeric silsesquioxane compound includes an amine or an alcohol
on an organic tether, which reacts with the carboxylic acid
attachment point to produce either an amide or an ester bond. The
amide or ester bond includes a carbonyl carbon directly connected
to a phenyl group in the polymer backbone. The resultant polyimide
polymer has many beneficial properties.
Inventors: |
Poe; Garrett (Madison, AL),
Farmer; Brandon (Huntsville, AL) |
Assignee: |
NeXolve Corporation
(Huntsville, AL)
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Family
ID: |
40460002 |
Appl.
No.: |
11/972,768 |
Filed: |
January 11, 2008 |
Prior Publication Data
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Document
Identifier |
Publication Date |
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US 20090069508 A1 |
Mar 12, 2009 |
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Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
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60970571 |
Sep 7, 2007 |
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Current U.S.
Class: |
525/418;
525/421 |
Current CPC
Class: |
C08F
290/065 (20130101); C08G 77/455 (20130101); C08F
290/145 (20130101); C08G 73/1039 (20130101); C08F
283/04 (20130101); C08G 73/1007 (20130101); C08G
77/045 (20130101) |
Current International
Class: |
C08F
283/04 (20060101) |
Field of
Search: |
;525/418,421 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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2004051848 |
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Feb 2004 |
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JP |
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2005232024 |
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Sep 2005 |
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JP |
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Other References
Yiwang Chen, Lie Chen, Huarong Nie, E.T. Kang, Low-.kappa.
Nanocomposite Films Based on Polyimides with Grafted Polyhedral
Oligomeric Silsesquioxane, Journal of Applied PolymerScience, vol.
99, 2226-2232 (2006), Wiley Periodicals, Inc. cited by other .
Amy L. Brunsvold, Timothy K. Minton, Irina Gouzman, Eitan Grossman,
Rene I. Gonzalez, An Investigation of the resistance of POSS
polyimide to atomic oxygen attack, Proceedings of the 9th
International Symposium on Materials in a Space Environment, Jun.
16-20, 2003, Noordwijk, The Netherlands. Compiled by K. Fletcher
ESA SP-540, Noordwijk, Netherlands: ESA Publications Division, ISBN
92-9092-850-6, 2003, p. 153-158. cited by other .
Chyi-Ming Leu, Yao-Te Chang, Kung-Hwa Wei, Synthesis and Dielectric
Properties of Polyimide-Tethered Polyhedral Oligomeric
Silsesquioxane (POSS) Nanocomposites via POSS-diamine,
Macromolucules 2003, 36, 9122-9127, 2003 American Chemical Society,
Published on Web Oct. 28, 2003. cited by other .
Michael E. Wright, Brian J. Petteys, Andrew J. Guenthner, Stephen
Fallis, Gregory R. Yandek, Sandra J. Tomczak, Timothy K. Minton,
Amy Brunsvold, Chemical Modification of Fluorinated Polyimides: New
Thermally Curing Hybrid Polymers with POSS, Macromolecules 2006,
39, 4710-4718, 2006 American Chemical Society, Published on Web
Jun. 17, 2006. cited by other .
Michael E. Wright, Stephen Fallis, Andy J. Guenthner, Lawrence C.
Baldwin, Synthesis of Hydroxymethyl-Functionalized Polyimides and
the Facile Attachment of an Organic Dye Utilizing Bis(isocyanates)
and Bis(acid chloride) Linkers, Macromolecules 2005, 38,
10014-10021, 2005 American Chemical Society, Published on Web Nov.
5, 2005. cited by other .
Chyi-Ming Leu, Yao-Te Chang, Kung-Hwa Wei, Polyimide-Side-Chain
Tethered Polyhedral Oligomeric Silsesquioxane Nanocomposites for
Low-Dielectric Film Applications, Chem.Mater. 2003, 15, 3721-3727,
2003 American Chemical Society, Published on Web Aug. 19, 2003.
cited by other .
Chia-Hung Chou, So-Lin Hsu, K. Dinakaran, Mao-Yuan Chiu, Kung-Hwa
Wei, Synthesis and Characterization of Luminescent Polyfluorenes
Incorporating Side-Chain-Tethered Polyhedral Oligomeric
Silsesquioxane Units, Macromolecules, 38 (3), 745-751, 2005
American Chemical Society, Web Release Date: Jan. 13, 2005. cited
by other.
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Primary Examiner: Gulakowski; Randy
Assistant Examiner: Loewe; Robert
Attorney, Agent or Firm: Swanson; Mark Mixon; David E.
Bradley Arant Boult Cummings LLP
Parent Case Text
The present application claims priority from provisional
application No. 60/970,571, filed Sep. 7, 2007. The content of that
application is hereby incorporated by reference.
Claims
What is claimed is:
1. A method of producing a polyimide polymer comprising: (a)
reacting at least one acid monomer with at least one diamino
monomer to form a polyimide polymer backbone such that the polymer
backbone includes at least-one non-terminal attachment point,
wherein the attachment point is comprised of a carboxylic acid; (b)
after step a), reacting a functional group selected from the group
consisting of an amine and an alcohol with the polymer backbone
attachment point, wherein the functional group is connected to an
oligomeric silsesquioxane (OS) compound.
2. The method of claim 1 wherein the polyimide polymer is
soluble.
3. The method of claim 1 wherein the polymer backbone is isolated a
maximum of one time.
4. The method of claim 1 wherein the OS is in the form of a
polyhedron.
5. The method of claim 4 wherein the acid monomer is
4-4'-(hexafluoroisopropylidene) diphthalic anhydride (6-FDA), and
wherein the diamino monomer is both diamino benzoic acid (DBA) and
para phenylene diamine (p-PDA).
6. A method of producing a polyimide polymer comprising: (a)
reacting at least one acid monomer with at least one diamino
monomer to form a polymer backbone, wherein the monomers are
selected such that the polymer backbone includes a non-terminal
attachment point; (b) imidizing the polymer from step (a) to form a
soluble polyimide polymer backbone, (c) after step (b), reacting an
oligomeric silsesquioxane (OS) compound to the attachment point on
the polyimide polymer backbone, such that the OS compound is
connected to the polyimide polymer backbone; and (d) isolating the
polymer backbone a maximum of one time.
7. The method of claim 6 wherein the OS is polyhedral and the OS
has the general formula (RSi).sub.n-1(R'A).sub.1(O.sub.1.5).sub.n
wherein R and R' are organic substituents, A is an element, n is an
natural number, 1 represents the number one, and wherein R' is
connected to the polymer backbone.
8. The method of claim 7 wherein A is selected from the group
consisting of Si, Al, B, Ge, Sn, Ti and Sb.
9. The method of claim 7 wherein the acid monomer is
4-4'-(hexafluoroisopropylidene) diphthalic anhydride (6-FDA),
wherein the diamino monomer is both diamino benzoic acid (DBA) and
para phenylene diamine (p-PDA), and wherein A is silicon.
10. The method of claim 6 wherein step (b) further comprises
removing water from the imidization reaction by vaporizing and
removing the water.
11. The method of claim 10 wherein step (c) further comprises
removing water from the OS attachment reaction by vaporizing and
removing the water.
12. The method of claim 6 wherein the attachment point is a
carboxylic acid, and the OS compound includes at least one
functional group selected from the group consisting of an amine and
an alcohol, wherein the function group reacts with the carboxylic
acid attachment point to connect the OS compound to the polymer
backbone.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to high performance polymers. In particular,
this invention relates to polyimide polymers, which have many
desirable properties, such as thermal stability and strength.
2. Description of the Related Art
Polyimides are an important class of polymeric materials and are
known for their superior performance characteristics. These
characteristics include high glass transition temperatures, good
mechanical strength, high Young's modulus, good UV durability, and
excellent thermal stability. Most polyimides are comprised of
relatively rigid molecular structures such as aromatic/cyclic
moieties.
As a result of their favorable characteristics, polyimide
compositions have become widely used many industries, including the
aerospace industry, the electronics industry and the
telecommunications industry. In the electronics industry, polyimide
compositions are used in applications such as forming protective
and stress buffer coatings for semiconductors, dielectric layers
for multilayer integrated circuits and multi-chip modules, high
temperature solder masks, bonding layers for multilayer circuits,
final passivating coatings on electronic devices, and the like. In
addition, polyimide compositions may form dielectric films in
electrical and electronic devices such as motors, capacitors,
semiconductors, printed circuit boards and other packaging
structures. Polyimide compositions may also serve as an interlayer
dielectric in both semiconductors and thin film multichip modules.
The low dielectric constant, low stress, high modulus, and inherent
ductility of polyimide compositions make them well suited for these
multiple layer applications. Other uses for polyimide compositions
include alignment and/or dielectric layers for displays, and as a
structural layer in micromachining applications.
In the aerospace industry, polyimide compositions are used for
optical applications as membrane reflectors and the like. In
application, a polyimide composition is secured by a metal (often
aluminum, copper, or stainless steel) or composite (often
graphite/epoxy or fiberglass) mounting ring that secures the border
of the polyimide compositions. Such optical applications may be
used in space, where the polyimide compositions and the mounting
ring are subject to repeated and drastic heating and cooling cycles
in orbit as the structure is exposed to alternating periods of
sunlight and shade.
Polyimide polymers are subject to rapid degradation in a highly
oxidizing environment, such as an oxygen plasma or atomic oxygen
[AO], as are most hydrocarbon- and halocarbon-based polymers. AO is
present in low earth orbit [LEO], so many spacecraft experience
this highly oxidizing environment. The interactions between the
oxidizing environment and the polymer material can erode and reduce
the thickness of the polymer material. To prevent the erosion,
protective coatings including metals, metal oxides, ceramics,
glasses, and other inorganic materials are often applied as surface
treatments to polyimides subjected to the oxidizing
environment.
While these coatings are effective at preventing the oxidative
degradation of the underlying material, they often experience
cracking from thermal and mechanical stresses, mechanical abrasion,
and debris impact. After cracking, the protective surface is
compromised and the underlying polymeric material can be degraded
from additional exposure to the oxidizing environment. Therefore,
the availability of polymers which are able to resist AO
degradation is very desirable.
Oligomeric silsesquioxanes [OS] can be incorporated into a
polyimide matrix to improve the durability of polyimides in these
environments. Polyimides with incorporated OS demonstrate excellent
resistance to AO degradation prevalent in LEO environments.
Polyimides with incorporated OS provide additional benefits as
well. Polyhedral OS are referred to by the trademark POSS.TM., and
are a common form of OS. There are examples of polyimide polymers
which have incorporated OS currently in existence.
The article by Leu et al., "Synthesis and Dielectric Properties of
Polyimide-Tethered Polyhedral Oligomeric Silsesquioxane (POSS)
Nanocomposites via POSS-diamine" (Macromolecules 2003, 36,
9122-9127 (2003)) describes a polyimide polymer having a polyhedral
OS group attached to the polymer backbone with an organic tether.
The polyhedral OS is incorporated into the polymer by attachment to
a diamine monomer. The diamine monomer requires three reaction and
purification steps to synthesize and purify, and an additional
reaction step to incorporate the monomer into the polyimide polymer
with pyromellitic dianhydride (PMDA) utilized as the dianhydride
monomer.
Wright et al. discloses a polyhedral OS containing polyimide
polymer in the article "Chemical Modification of Fluorinated
Polyimides: New Thermally Curing Hybrid Polymers with POSS"
(Macromolecules 2006, 39, 4710-4718 (2006)). The polyhedral OS is
connected to the polymer backbone with an organic tether, and it is
connected through an available alcohol group on the diamine monomer
3,5-bis(4-aminophenoxy)-1-hydroxymethylbenzene (BNB).
4,4'-(hexafluoroisopropylidene)diphthalic anhydride (6-FDA) is used
as the dianhydride monomer, and the resulting polyimide polymer is
soluble in certain organic solvents. The method described for
creating the final polymer included isolating the polyimide polymer
three times: 1) after the polyimide polymer was formed; 2) after
the available alcohol group was modified to provide an acid
chloride; and 3) after the polyhedral OS was attached to the acid
chloride. Additionally, the BHB monomer was prepared in the lab,
because this compound is not commercially available.
Lichtenhan et al., in U.S. Pat. No. 6,933,345, describes many
polymers including polyhedral OS groups. Lichtenhan describes the
polyhedral OS being blended with the polymer, being reacted into
the polymer backbone, or being tethered to the polymer backbone.
The specific characteristics of the polymer are not disclosed, and
no method for producing a polyimide polymer with a polyhedral OS
group attached through an organic tether is disclosed.
US Patent Application 2006/0122350 by Wei et al. discloses a
polyimide polymer with a polyhedral OS group attached to the
polymer backbone with an organic tether. The polyhedral OS group is
attached through either an alkyl carbon or through a benzene ring
connected into the polymer backbone at the 1st and 4th carbon. If
the polyhedral OS group is attached to the polymer backbone at a
benzene ring, that benzene ring does not connect directly to two
imide groups. The method for preparing the polymer with the
polyhedral OS group includes forming a monomer with the polyhedral
OS group attached, and then making the polymer, or the polyimide
was created and chlorinated POSS was reacted with the polyimide.
The described process includes 2 isolations and purifications of
the polyimide polymer.
Svejda et al., in U.S. Pat. No. 6,767,930 describes incorporation
of polyhedral OS groups in polymers in general. The polyhedral OS
is incorporated by non-reactive blending, reactive grafting,
reactive polymerization into the polymer backbone, and reactive
cross-linking. The specific use of polyhedral OS groups in
polyimide polymers includes both non-reactively blending the
polyhedral OS group with a polyimide polymer, and reactively
polymerizing the polyhedral OS with the polyimide such that the
polyhedral OS group forms part of the polymer backbone.
Polyimide polymers have many desirable characteristics. Some
characteristics of polyimide polymers can be improved by the
incorporation of OS.
SUMMARY OF THE INVENTION
The current invention includes a polyimide polymer with a polymeric
backbone. An OS compound is attached to the polymeric backbone with
a tether. Therefore, the OS group is not incorporated into the
polymeric backbone. The current invention also includes a method
for producing the polyimide polymer herein described.
Other aspects and advantages of the invention will be apparent from
the following ion and the appended claims.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 depicts the formation of an amic acid.
FIG. 2 depicts the formation of an imide bond from an amic
acid.
FIG. 3 depicts an oligomeric silsesquioxane (OS) compound.
FIG. 4 depicts a polyhedral shaped OS compound.
FIG. 5 depicts 4-4'-[hexafluoroisopropylidene]diphthalic anhydride
[6-FDA]
FIG. 6 depicts 4,4'-oxydiphthalic anhydride [ODPA]
FIG. 7 depicts 1,3-diaminobenzoic acid [DBA]
FIG. 8 depicts 4,4'-diaminobiphenyl-3,3'-dicarboxylic acid
[DBDA].
FIG. 9 depicts para-phenylene diamine [p-PDA]
FIG. 10 depicts the formation of a tethering amide bond by the
reaction of an amine with a carboxylic acid.
FIG. 11 depicts a section of a representative polyimide polymer
with the OS connected with an amide.
FIG. 12 depicts a section of a representative polyimide polymer
with the OS connected with an ester.
FIG. 13 depicts a non-terminal phenyl group of the polymer
backbone, with an OS compound labeled X connected with an ester or
amide linkage, labeled XX.
FIG. 14 depicts different structures that represent the symbol YY
in FIG. 13, with ZZ representing a direct bond, --O--, --S--,
--SO--, --SO.sub.2--, --CH.sub.2--, --CF.sub.2--,
--C(CH.sub.3).sub.2--, --(CF.sub.3).sub.2--, --(CH.sub.2).sub.n--,
--(CH.sub.2CHCH.sub.3O).sub.n--, --((CH.sub.2).sub.4O).sub.n--,
--(Si(CH.sub.3).sub.2O).sub.n--, --(SiH(CH.sub.3)O).sub.n--,
--(SiH(C.sub.6H.sub.5)O).sub.n--, or
--(Si(C.sub.6H.sub.5).sub.2O).sub.n--.
FIG. 15 depicts possible structures which are represented by the
"WW" in FIG. 14.
NOTE: The use of waved lines indicates the molecule continues, but
does not necessarily repeat. The use of square brackets "[" and/or
"]" indicates that the structure repeats beyond the bracket. The
use of round brackets "(" and/or ")" indicates substructures within
a repeat unit and does not indicate the substructure repeats beyond
the round brackets. In this description, an atom AA shown connected
to a phenyl group through a bond, instead of at the angles
representing carbon atoms, is meant to depict the atom AA connects
to any available carbon atom in the phenyl group, and not to a
specific carbon atom. Therefore, such a drawing does not
specifically denote an ortho, meta, or para positioning of the bond
to the AA atom.
DETAILED DESCRIPTION
Polyimide
Polyimides are a type of polymer with many desirable
characteristics. In general, polyimide polymers include a nitrogen
atom in the polymer backbone, wherein the nitrogen atom is
connected to two carbonyl carbons, such that the nitrogen atom is
somewhat stabilized by the adjacent carbonyl groups. A carbonyl
group includes a carbon, referred to as a carbonyl carbon, which is
double bonded to an oxygen atom. Most polyimides are considered an
AA-BB type polymer because two different classes of monomers are
used to produce the polyimide polymer. One class of monomer is
called an acid monomer, and is usually in the form of a
dianhydride. The other type of monomer is usually a diamine, or a
diamino monomer. Polyimides may be synthesized by several methods.
In the traditional two-step method of synthesizing aromatic
polyimides, a polar aprotic solvent such as N-methylpyrrolidone
(NMP) is used. First, the diamino monomer is dissolved in the
solvent, and then a dianhydride monomer is added to this solution.
The diamine and the acid monomer are generally added in
approximately a 1:1 molar stoichiometry.
Because one dianhydride monomer has two anhydride groups, different
diamino monomers can react with each anhydride group so the
dianhydride monomer may become located between two different
diamino monomers. The diamine monomer contains two amine functional
groups; therefore, after one amine attaches to the first
dianhydride monomer, the second amine is still available to attach
to another dianhydride monomer, which then attaches to another
diamine monomer, and so on. In this matter, the polymer backbone is
formed. The resulting polycondensation reaction forms a polyamic
acid. The reaction of an anhydride with an amine to form an amic
acid is depicted in FIG. 1. The high molecular weight polyamic acid
produced is soluble in the reaction solvent and, thus, the solution
may be cast into a film on a suitable substrate such as by flow
casting. The cast film can be heated to elevated temperatures in
stages to remove solvent and convert the amic acid groups to imides
with a cyclodehydration reaction, also called imidization.
Alternatively, some polyamic acids may be converted in solution to
soluble polyimides by using a chemical dehydrating agent, catalyst,
and/or heat. The conversion of an amic acid to an imide is shown in
FIG. 2.
The polyimide polymer is usually formed from two different types of
monomers, and it is possible to mix different varieties of each
type of monomer. Therefore, one, two, or more dianhydride-type
monomers can be included in the reaction vessel, as well as one,
two or more diamino monomers. The total molar quantity of
dianhydride-type monomers is kept about the same as the total molar
quantity of diamino monomers. Because more than one type of diamine
or dianhydride can be used, the exact form of each polymer chain
can be varied to produce polyimides with desirable properties.
For example, a single diamine monomer AA can be reacted with two
dianhydride comonomers, B.sub.1B.sub.1 and B.sub.2B.sub.2, to form
a polymer chain of the general form of
(AA-B.sub.1B.sub.1).sub.x-(AA-B.sub.2B.sub.2).sub.y in which x and
y are determined by the relative incorporations of B.sub.1B.sub.1
and B.sub.2B.sub.2 into the polymer backbone. Alternatively,
diamine comonomers A.sub.1A.sub.1 and A.sub.2A.sub.2 can be reacted
with a single dianhydride monomer BB to form a polymer chain of the
general form of
(A.sub.1A.sub.1-BB).sub.x-(A.sub.2A.sub.2-BB).sub.y. Additionally,
two diamine comonomers A.sub.1A.sub.1 and A.sub.2A.sub.2 can be
reacted with two dianhydride comonomers B.sub.1B.sub.1 and
B.sub.2B.sub.2 to form a polymer chain of the general form
(A.sub.1A.sub.1-B.sub.1B.sub.1).sub.w-(A.sub.1A.sub.1-B.sub.2B.sub.2).sub-
.x-(A.sub.2A.sub.2-B.sub.1B.sub.1).sub.y-(A.sub.2A.sub.2-B.sub.2B.sub.2).s-
ub.z, where w, x, y, and z are determined by the relative
incorporation of A.sub.1A.sub.1-B.sub.1B.sub.1,
A.sub.1A.sub.1-B.sub.2B.sub.2, A.sub.2A.sub.2-B.sub.1B.sub.1, and
A.sub.2A.sub.2-B.sub.2B.sub.2 into the polymer backbone. Therefore,
one or more diamine monomers can be polymerized with one or more
dianhydrides, and the general form of the polymer is determined by
varying the amount and types of monomers used.
The dianhydride is only one type of acid monomer used in the
production of AA-BB type polyimides. It is possible to used
different acid monomers in place of the dianhydride. For example, a
tetracarboxylic acid with four acid functionalities, a tetraester,
a diester acid, or a trimethylsilyl ester could be used in place of
the dianhydride. In this description, an acid monomer refers to
either a dianhydride, a tetraester, a diester acid, a
tetracarboxylic acid, or a trimethylsilyl ester. The other monomer
is usually a diamine, but can also be a diisocyanate. Polyimides
can also be prepared from AB type monomers. For example, an
aminodicarboxylic acid monomer can be polymerized to form an AB
type polyimide.
The characteristics of the polyimide polymer are determined, at
least in part, by the monomers used in the preparation of the
polymer. The proper selection and ratio of monomers are used to
provide the desired polymer characteristics. For example,
polyimides can be rendered soluble in organic solvents by selecting
the monomers that impart solubility into the polyimide structure.
It is possible to produce a soluble polyimide polymer using some
monomers that tend to form insoluble polymers if the use of the
insoluble monomers is balanced with the use of sufficient
quantities of soluble monomers, or through the use of lower
quantities of especially soluble monomers. The term especially
soluble monomers refers to monomers which impart more of the
solubility characteristic to a polyimide polymer than most other
monomers. Some soluble polyimide polymers are soluble in relatively
polar solvents, such as dimethylacetamide, dimethylformamide,
dimethylsulfoxide, tetrahydrofuran, acetone, methyl ethyl ketone,
methyl isobutyl ketone, and phenols, as well as less polar
solvents, including chloroform, and dichloromethane. The solubility
characteristics and concentrations of the selected monomers
determine the solubility characteristics of the resultant polymer.
For this description, a polymer is soluble if it can be dissolved
in a solvent to form at least a 1 percent solution of polymer in
solvent, or more preferably a 5 percent solution, and most
preferably a 10 percent or higher solution.
Most, but not all, of the monomers used to produce polyimide
polymers include aromatic groups. These aromatic groups can be used
to provide an attachment point on the polymer backbone for a
tether. A tether refers to a chain including at least one carbon,
oxygen, sulfur, phosphorous, or silicon atom that is used to
connect the polymer backbone to another compound or sub-compound.
Therefore, if the polymer backbone were connected through the para
position on a phenyl group, wherein the para position refers to the
number 1, and the number 4 carbons on the benzene ring, the ortho
and meta positions would be available to attach a tether to this
polymer backbone. The ortho position to the number 1 carbon refers
to the number 2 and number 6 carbons, whereas the meta position to
the number 1 carbon refers to the number 3 and number 5
carbons.
Many polyimide polymers are produced by preparing a polyamic acid
polymer in the reaction vessel. The polyamic acid is then formed
into a sheet or a film and subsequently processed with heat (often
temperatures higher than 250 degrees Celsius) or both heat and
catalysts to convert the polyamic acid to a polyimide. However,
polyamic acids are moisture sensitive, and care must be taken to
avoid the uptake of water into the polymer solution. Additionally,
polyamic acids exhibit self-imidization in solution as they
gradually convert to the polyimide structure. The imidization
reaction generally reduces the polymer solubility and produces
water as a by-product. The water produced can then react with the
remaining polyamic acid, thereby cleaving the polymer chain.
Moreover, the polyamic acids can generally not be isolated as a
stable pure polymer powder. As a result, polyamic acids have a
limited shelf life.
Sometimes it is desirable to produce the materials for a polyimide
polymer film, but wait for a period of time before actually casting
the film. For this purpose, it is possible to store either a
soluble polyimide or a polyamic acid. Soluble polyimides have many
desirable advantages over polyamic acids for storage purposes.
Soluble polyimides are in general significantly more stable to
hydrolysis than polyamic acids, so the polyimide can be stored in
solution or it can be isolated by a precipitation step and stored
as a solid material for extended periods of time. If a polyamic
acid is stored, it will gradually convert to the polyimide state
and/or hydrolytically depolymerize. If the stored material becomes
hydrolytically depolyermized, it will exhibit a reduction in
solution viscosity, and if the stored material converts to the
polyimide state, it will become gel-like or a precipitated solid if
the polyimide is not soluble in the reaction medium. This reduced
viscosity solution may not exhibit sufficient viscosity to form a
desired shape, and the gel-like or solid material cannot be formed
to a desired shape. The gradual conversion of the polyamic acid to
the polyimide state generates water as a byproduct, and the water
tends to cleave the remaining polyamic acid units. The cleaving of
the remaining polyamic acid units by the water is the hydrolytic
depolymerization referred to above. Therefore, the production of
soluble polyimides is desirable if there will be a delay before the
material is formed for final use.
Soluble polyimides have advantages over polyamic acids besides
shelf life. Soluble polyimides can be processed into usable work
pieces without subjecting them to the same degree of heating as is
generally required for polyamic acids. This allows soluble
polyimides to be processed into more complex shapes than polyamic
acids, and to be processed with materials that are not durable to
the 250 degree Celsius minimum temperature typically required for
imidizing polyamic acids. To form a soluble polyimide into a
desired film, the polyimide is dissolved in a suitable solvent,
formed into the film as desired, and then the solvent is
evaporated. The film solvent can be heated to expedite the
evaporation of the solvent.
Oligomeric Silsesquioxane
OS compounds or groups are characterized by having the general
formula of [RSi].sub.n[O.sub.1.5].sub.n wherein the R represents an
organic substituent and the Si and the O represent the chemical
symbols for the elements silicon and oxygen. R can be aliphatic or
aromatic, and includes a wide variety of organic compounds. The
silicon atoms are connected together through the oxygen atoms, with
the R groups connected to the silicon atoms, as seen in FIG. 3.
These OS compounds have hybrid organic and inorganic properties.
The Si--O groupings provide the inorganic properties, and the
attached R groups provide the organic properties. Frequently, these
OS compounds exist in a cage form such that a polyhedron is created
by the silicon and oxygen atoms, as shown in FIG. 4. When the OS
compound is in the cage form or the polyhedral form, the R groups
are exterior to the cage, with the Si atoms generally forming
corners of the cage.
Frequently, the OS compound will have an organic substituent which
has a functional group. These OS compounds can therefore have
organic substituents with varying structures connected to the
different Si atoms within a single OS compound. A typical example
would be a polyhedral OS represented by the formula
[RSi].sub.(n-1)[R'A].sub.1[O.sub.1.5].sub.n, wherein R' symbolizes
an organic substituent with a functional group which can be used to
connection the OS compound to a polymer backbone or some other
molecule. In this case, the A is used to represent an element. This
element is usually Si, but can also be other elements, including
aluminum (Al), boron (B), germanium (Ge), tin (Sn), titanium (Ti),
and antimony (Sb). These different atoms incorporated into the OS
compound provide different characteristics which will be imparted
to the polymer.
Attaching an OS group to a polyimide polymer can affect many
characteristics of the polymer, including oxidative stability,
temperature stability, glass transition temperature, solubility,
dielectric constant, tensile properties, thermomechanical
properties, optical properties, and other properties. One
significant characteristic improved by incorporation of OS in a
polyimide polymer is increased resistance to degradation in
oxidizing environments, such as oxygen plasma and AO, as discussed
above. Oligomeric silsesquioxanes [OS] can be incorporated into a
polyimide matrix to improve the durability of polyimides in these
environments. Therefore, polyimide polymers with incorporated OS
are desirable.
When a hydrocarbon or halocarbon polyimide that includes OS is
exposed to an oxygen plasma or AO, the organic substituent portions
of the OS oxidize into volatile products while the inorganic
portion forms a passivating silica layer on the exposed surface of
the polyimide polymer. This process is referred to as the
glassification of the polymer. The silica layer tends to protect
the underlying polyimide material from further degradation by the
oxidizing environment. Additionally, the silica layer absorbs at
least a portion of the ultraviolet [UV] light, especially the UV
light with a wavelength shorter than about 256 nm. Therefore, the
silica layer also protects the polymer film from radiation, because
UV light is a form of electromagnetic radiation. Additionally, if
the silica layer exhibits sufficient thickness, it reduces gas and
water permeability through the polyimide film. It is possible to
deposit additional silica on a polyimide film to produce the
thickness necessary to significantly reduce gas and water
permeability through the film.
OS has been blended with polymers to provide a degree of protection
herein described, but the amount of OS which can be blended with a
polymer is limited. Typically, the amount of OS incorporated into
the polymer by blending methods is limited to a concentration where
the OS compounds do not aggregate into domains large enough to
scatter visible light. Incorporation of additional OS above this
level typically results in a reduction in optical and/or mechanical
properties. However, it has been found that chemically reacting the
OS with the polymer reduces the OS aggregation and provides more
uniform distribution of the OS within the polymer. As such, more OS
can typically be incorporated into a polymer matrix via covalent
bonding than by simpler blending methods. This results in a polymer
which is better able to withstand exposure to oxygen plasma, AO,
and UV radiation.
It is possible to attach the OS groups to a polyimide polymer by
reacting the OS with one of the monomers before polymerization. In
practice, however, this method is difficult owing to the high cost
of OS and number of reaction and purification steps needed to
obtain a usable monomer of sufficient purity. High monomer purity
is required for the formation of sufficient molecular weight
polyamic acids and polyimides. For example, Leu et al. (as
discussed above) describe a method to incorporate a polyhedral OS
into the polyimide backbone requiring four reaction steps total.
The fewer the reaction and purification steps, the lower the cost
and the greater the efficiency of the entire process, as a general
rule. Therefore, a method of producing a desired polymer with fewer
reaction and purification steps is desirable, because such a method
would probably reduce the cost and improve the overall efficiency
of the process. The current disclosure describes a polyimide
composition incorporating OS and a method of synthesizing that
polymer that uses two reaction steps and zero to two purification
steps.
Selection of Monomers
The characteristics of the final polymer are largely determined by
the choice of monomers which are used to produce the polymer.
Factors to be considered when selecting monomers include the
characteristics of the final polymer, such as the solubility,
thermal stability and the glass transition temperature. Other
factors to be considered include the expense and availability of
the monomers chosen. Commercially available monomers that are
produced in large quantities generally decrease the cost of
producing the polyimide polymer film since such monomers are in
general less expensive than monomers produced on a lab scale and
pilot scale. Additionally, the use of commercially available
monomers improves the overall reaction efficiency because
additional reaction steps are not required to produce a monomer
which is incorporated into the polymer. One advantage of the
current invention is the preferred monomers are generally produced
in commercially available quantities, which can be greater than
10,000 kg per year.
One type of monomer used is referred to as the acid monomer, which
can be either the tetracarboxylic acid, tetraester, diester acid, a
trimethylsilyl ester, or dianhydride. The use of the dianhydride is
preferred because it generally exhibits higher rates of reactivity
with diamines than tetrafunctional acids, diester acids,
tetraesters, or trimethylsilyl esters. Some characteristics to be
considered when selecting the dianhydride monomer include the
solubility of the final polymer as well as commercial availability
of the monomers.
Certain characteristics tend to improve the solubility of the
polyimide polymer. These characteristics include flexible spacers,
so-called kinked linkages, and bulky substituents. The flexible
spacer is an atom covalently bonded between two separate phenyl
groups. The phenyl groups are relatively ridged, so the flexible
spacer allows for increased motion between separate phenyl groups.
Alkyl linkages are not as stable as the phenyl groups, so the use
of simple hydrocarbon alkyl groups between phenyl groups can reduce
the stability of the polymer. The stability of the overall polymer
can be improved if the linkage is saturated with fluorine instead
of hydrogen. Also, the use of other atoms, such as Oxygen or
Silicon, can result in a more stable polymer.
The term kinked linkages refers to a meta connection on a phenyl
group. This means the polymer backbone would be connected through a
number 1 and number 3 carbon on a phenyl group. The use of kinked
linkages, or meta linkages, in the polymer backbone tends to result
in a higher coefficient of thermal expansion, as well as greater
solubility.
Bulky substituents in the polymer also tend to increase the overall
polymer solubility. Bulky substituents are compounds which are
large and tend to interfere with intramolecular and intermolecular
chain association because of their size. The bulky substituents can
be included between phenyl groups in the backbone, connected
directly to a phenyl group, or they can be tethered to the polymer
backbone. The bulky substituents tend to reduce the ability of
adjacent polymer chains to tightly associate. This tends to allow
solvent molecules to enter between adjacent polymer chains, which
increases the polymer solubility.
Urging phenyl groups in the backbone to align in different planes
also tends to increase the polymer solubility, and bulky
substituents can be used for this purpose. If two phenyl groups in
the backbone are relatively close together, and each has a bulky
substituent connected or tethered to it, those bulky substituents
will sterically interfere with each other and tend to urge one
phenyl group into a plane that is perpendicular to the plane of the
other phenyl group.
The flexible spacers can be saturated with larger components such
as fluorine to improve the solubility of the resulting polyimide
polymer. Other preferred atoms for separating the phenyl groups
include oxygen and sulfur. The preferred dianhydride monomers of
the current invention are 6-FDA and ODPA, as seen in FIGS. 5 and 6,
but other dianhydride monomers may also be used.
The OS group is usually attached to the diamine monomer owing to
the greater availability of diamine architectures as compared to
dianhydrides. Therefore, one of the diamine monomers used should
have a functional group independent of the two amines, so when the
two amines are incorporated into the polymer backbone the
functional group is still available for a subsequent chemical
reaction. This functional group is referred to as an attachment
point because this is the point where the OS group is attached to
the polymer backbone. A wide variety of attachment points can be
used, but a carboxylic acid is preferred. The attachment point is
incorporated into the length of the polymer backbone, not just at
the ends or terminus of each chain, so that more OS compounds can
be attached to the polymer. Therefore, the preferred attachment
point is a carboxylic acid connected by a single bond to a phenyl
group, wherein the phenyl group is non-terminal and the phenyl
group is part of the polymer backbone.
The monomers DBA and DBDA, as shown in FIGS. 7 and 8, are utilized
heavily because of the presence of a carboxylic acid group, which
serve as attachment points for the OS groups. Other diamino
monomers without a free attachment point can also be used. One
example of such a monomer is p-PDA, as seen in FIG. 9. The higher
the concentration of diamino monomers without an attachment point,
the lower the concentration of diamino monomers with a free
attachment point. Since the OS is primarily incorporated into the
polymer through the attachment point, the overall concentration of
OS compounds in the final polymer can be controlled by varying the
ratio of diamino monomers with and without a free attachment point.
Many other diamino monomers can be used, including but not limited
to 2,2-bis[4-(4-aminophenoxy)phenyl]hexafluoropropane (BDAF), 1,3
bis(3-aminophenoxy) benzene (APB), 3,3-diaminodiphenyl sulfone
(3,3-DDSO2), 4,4'-diaminodiphenylsulfone (4,4'-DDSO2), meta
phenylene diamine (m-PDA), oxydianiline (ODA), the isomers of
4,4'-Methylenebis(2-methylcyclohexylamine) (MBMCHA), the isomers of
4,4'-Methylenebis(cyclohexylamine) (MBCHA), the isomers of
1,4-cyclohexyldiamine (CHDA), 1,2-diaminoethane,
1,3-diaminopropane, 1,4-diamonbutane, 1,5-diaminopentane,
1,6-diaminohexane, and diamonodurene (DMDE).
Any remaining free carboxylic acid groups at the end or terminus of
a polymer chain also serve as attachment points. These terminal
carboxylic acid groups are generally connected to a terminal phenyl
group. Attachment points connected to non-terminal phenyl groups
are needed to increase the ratio of OS incorporation into the final
polymer. A non-terminal phenyl group is a phenyl group in the
polymer backbone which is between two imide bonds in the polymer
backbone.
Oligomeric Silsesquioxane Considerations
Incorporation of the OS group into the polymer matrix is often
beneficial for the reasons previously described. Usually, the OS
group is incorporated in a polyhedral cage structure, and the
polyhedral OS is attached to the polymer. In such instances, the OS
group will have at least one organic substituent with a functional
group for attaching to the polymer. It is also possible for the
polyhedral OS to have two or more functional groups, in which case
it can be used as a monomer or as a crosslinking component. For
example, a polyhedral OS group including two amine functional
groups could be incorporated into the backbone of the polymer as a
diamine monomer. Alternatively, if one OS functional group were to
attach to a first polymer chain, the second OS functional group
could attach to a different polymer chain, so the two polymer
chains were cross-linked. The polymer chains can also be
crosslinked by the inclusion of other organic substituents which
will crosslink the polymer chains.
The current invention considers the attachment of the OS groups to
the backbone by a tether such that these OS groups do not form part
of the backbone of the polymer. This provides advantages as
compared to incorporating the OS groups in line with the polymer
backbone. One advantage of pendant attachment is that OS and
polyhedral OS are rigid. When rigid OS is incorporated in line with
the polymer backbone, it generally increases the material stiffness
and ultimately limits the amount of polyhedral OS incorporated. By
attaching the OS groups to the backbone by a tether, the bulk
polymer stiffness is less affected with increasing levels of OS
incorporation.
It is advantageous to use a tether when attaching an OS group to a
polyimide polymer backbone. Steric factors favor the use of a
tether because the OS group is usually bulky and the tether
provides an attachment point apart from the bulk of the OS. The
tether needs to be long enough to allow the OS group to react with
the polyimide backbone, but it should be short enough to restrict
the OS from aggregating into large domains that reduce the
mechanical and optical properties. Keeping the OS group close to
the attachment point provides for the OS being relatively evenly
distributed throughout the polymer. Also, keeping the OS group
close to the attachment point provides for a bulky constituent near
the polymer backbone, which tends to improve the solubility of the
polymer. Some desired properties of the tether itself include the
provision of a stable and secure attachment, as well as many
degrees of freedom of movement for the OS group. The tether can
include functional groups, such as amides or esters. A tether or
chain about 5 atoms long provides enough length to allow attachment
while keeping the OS group close enough to the attachment
point.
Preferably, the OS compound will include one organic substituent
with a functional group for attachment to the polymer backbone.
This organic substituent becomes the tether connecting the polymer
backbone to the OS compound. The preferred functional groups for
this attachment are either an amine or an alcohol. The amine or
alcohol on the OS compound reacts with a carboxylic acid on the
polymer backbone to form an amide or ester connection, with the
amide reaction shown in FIG. 10. The amide or ester thus formed
includes either a carbonyl carbon and a linking nitrogen, as seen
in FIGS. 10 and 11, or a carbonyl carbon and a linking oxygen, as
seen in FIG. 12. The carbonyl carbon is connected by a single bond
to a phenyl group, and the phenyl group is part of the polymer
backbone. The linking nitrogen or oxygen is connected by a single
bond to the carbonyl carbon, and the linking nitrogen or oxygen is
also connected to the organic substituent tether for the OS
compound.
By using the process described above, a polyimide polymer is formed
which contains either an amide or ester linking a non-terminal
phenyl group to the OS compound. This section of the polymer is
shown in FIG. 13. The nitrogen in FIG. 13 is from the imide bond in
the polymer backbone, XX represents either an oxygen atom or a
nitrogen atom with an attached hydrogen, depending on if the OS
group is attached by forming an ester or an amide, respectively.
The symbol X represents an OS compound, including a tether between
the XX atom and the OS compound. FIG. 14 depicts different
structures that represent the symbol YY in FIG. 13. These different
structures depend on the diamino monomers selected for the
polyimide polymer. The symbol WW in FIG. 14 represents either a
hydrogen atom, a free carboxylic acid, or another amide or ester
linkage with an attached OS compound, as shown in FIG. 15, wherein
XX again represents either nitrogen with an attached hydrogen or
oxygen. The symbol ZZ in FIG. 14 represents a direct bond, --O--,
--S--, --SO--, --SO.sub.2--, --CH.sub.2--, --CF.sub.2--,
--C(CH.sub.3).sub.2--, --(CF.sub.3).sub.2--, --(CH.sub.2).sub.n--,
--(CH.sub.2CHCH.sub.3O).sub.n--, --((CH.sub.2).sub.4O).sub.n--,
--(Si(CH.sub.3).sub.2O).sub.n--, --(SiH(CH.sub.3)O).sub.n--,
--(SiH(C.sub.6H.sub.5)O).sub.n--, or
--(Si(C.sub.6H.sub.5).sub.2O).sub.n--. In these Figs., an atom
shown connected to a phenyl group through a bond, instead of at the
hexagon angles representing carbon atoms, is meant to depict that
atom connected to any available carbon atom in the phenyl group,
and not to a specific carbon atom.
Process
The process for creating the final polymer should involve as few
reactions and as few isolations as possible to maximize the overall
efficiency. Minimization of the number of vessels or pots which are
used during the production process also tends to improve
efficiency, because this tends to minimize the number of reactions
and/or isolations of the polymer.
The first step is forming the polyimide polymer backbone. Some of
the basic requirements for this polymer backbone are that it be
soluble, that it include an attachment point, and that it has many
of the desirable characteristics typical of polyimide polymers.
Typically, the diamino monomers will be dissolved in a solvent,
such as dimethylacetamide [DMAc]. After the diamino monomers are
completely dissolved, the dianhydride monomer is added to the
vessel and allowed to react for approximately 4 to 24 hours. The
use of an end capping agent, such as a monoanhydride or a
monoamine, is not preferred until after the polymerization reaction
is allowed to proceed to completion. At that point, the addition of
phthalic anhydride or other monoanhydride end-capping agents can be
used to react with remaining end group amines. Adding end capping
agents during the polymerization reaction tends to shorten the
polymer chains formed, which can reduce desirable mechanical
properties of the resultant polymer. For example, adding end
capping agents during the polymerization reaction can result in a
more brittle polymer.
At this point the monomers have reacted together to form a polyamic
acid. It is desired to convert the polyamic acid to a polyimide.
The conversion of the polyamic acid to the polyimide form is known
as imidization, and is a condensation reaction which produces
water, as seen in FIG. 2. Because water is a by-product of a
condensation reaction, and reactions proceed to an equilibrium
point, the removal of water from the reaction system pushes or
drives the equilibrium further towards a complete reaction because
the effective concentration of the by-product water is reduced.
This is true for chemical reactions generally, including
condensation reactions.
The water can be removed from the reaction vessel chemically by the
use of anhydrides, such as acetic anhydride, or other materials
which will react with the water and prevent it from affecting the
imidization of the polyamic acid. Water can also be removed by
evaporation. One imidization method involves the use of a catalyst
to chemically convert the polyamic acid to the polyimide form. A
tertiary amine such as pyridine, triethyl amine, and/or
beta-picolline is frequently used as the catalyst. Another method
previously discussed involves forming the polyamic acid into a film
which is subsequently heated. This will vaporize water as it is
formed, and imidize the polymer.
A third imidization method involves removing the formed water via
azeotropic distillation. The polymer is heated in the presence of a
small amount of catalyst, such as isoquinoline, and in the presence
of an aqueous azeotroping agent, such as xylene, to affect the
imidization. The method of azeotropic distillation involves heating
the reaction vessel so that the azeotroping agent and the water
distill from the reaction vessel as an azeotrope. After the
azeotrope is vaporized and exits the reaction vessel, it is
condensed and the liquid azeotroping agent and water are collected.
If xylene, toluene, or some other compound which is immiscible with
water is used as the azeotroping agent, it is possible to separate
this condensed azeotrope, split off the water for disposal, and
return the azeotroping agent back to the reacting vessel.
If water is removed from the reaction vessel as a vapor it is
possible to proceed with the addition of the OS group without
isolating the polyimide polymer formed. However, if the water is
chemically removed, it is be preferred to precipitate, filter, and
possibly wash the polyimide before adding the OS group. If the
polymer is precipitated, the filtered solid polyimide is used as a
feed stock in the next step. It should be noted that when reference
is made to isolating a polyimide, it refers to the isolation of a
polyimide at one point in the entire production process. For
example, if a polyimide was formed from the basic monomers, then
precipitated and filtered, and then washed several times, this
would count as a single isolation and purification, even though
several washes were performed. Two isolations would be separated by
a reaction step.
The OS group is connected to the polyimide back bone previously
formed. One of the first steps is finding a suitable solvent to
dissolve the OS group as well as the polyimide polymer. Some
examples of suitable solvents include: methylene chloride,
chloroform, and possibly tetrahydrofuran (THF). One way to control
the amount of OS that is incorporated into the polymer is by
controlling the amount of monomer with an attachment point which
was included in the formation of the polyimide polymer. Another way
would be to limit the amount of OS added. If OS is added at a
stoichiometric quantity lower than the available attachment points,
then there would be free attachment points remaining. These free
attachment points could be used for cross linking with other
polymer chains, or they could be used for other purposes.
Preferably, the OS group is a monofunctional group with one
functional group for attachment to the polymer back bone.
Preferably this functional group is an amine or alcohol, which will
combine with the preferred carboxylic acid that is available as the
attachment point on the polymer back bone.
If the OS group includes two or more functional groups, different
polymer chains can be cross linked through this OS group. For this
to happen, the OS group would be linked to two separate polymer
chains. An amine on the OS group can be reacted with the carboxylic
acid on the polymer chain to form an amide bond. This is a
condensation reaction in which water is produced as a byproduct. In
order to drive the reaction to completion it is desirable for the
water to be removed from the reaction system. This can be
facilitated with a dehydrating agent, such as
dicyclohexylcarbodiimide, followed by subsequent isolation of the
polyimide polymer. Additionally, this amide-forming reaction can be
facilitated with an acid catalyst. If the OS compound contains an
alcohol for reacting with the carboxylic acid, water is still
produced as a byproduct and removing the water will tend to drive
the reaction to completion.
An alternate possibility is to remove the water via azeotropic
distillation from the reaction vessel. This can be done by adding
or by continuing to use an azeotroping agent such as xylene or
toluene, then vaporizing the water, separating the water from the
reaction vessel, and discarding the water after it has exited the
reaction vessel. This is similar to the process described above for
the imidization reaction.
The current process includes up to two isolations of the polyimide
polymer. The first possible isolation is after the polymer
imidization reaction, and the second possible isolation is after
the OS group has been attached to the polyimide polymer. The
azeotropic removal of water in the vapor form during a condensation
reaction eliminates the need for a subsequent isolation. Therefore,
if vaporous water is azeotropically removed during one of either
the polymer imidization reaction or the OS attachment reaction, the
number of isolations needed for the production of the final OS
containing polymer is reduced to one. If vaporous water is removed
after both of the above reactions, it is possible to produce the
final product with no isolations.
By using a carboxylic acid as the attachment point on the polyimide
polymer and an amine or an alcohol as the functional group on the
OS, it is possible to connect the OS group to the polymer back bone
using gentle reaction conditions. For example, the reaction
conditions for a carboxylic acid/amine attachment do not require
rigorous material purity or dryness, and/or could be completed at
the relatively low temperature of approximately 25 degrees Celsius.
Similar conditions are possible if an alcohol is reacted with the
carboxylic acid attachment point.
It should be noted that the cost of the OS compounds can be high
and so the use of a very efficient means for connecting the OS
group to the polyimide polymer is desired. The carboxylic acid and
amine group connection is very efficient in terms of OS attachment
to the polyimide, and can be utilized to produce connection
efficiencies in the range of about 73% to about 99%. After the OS
group has been connected to the polyimide polymer, the product can
be stored for use at a later time or it can be used immediately to
produce polymer films or other desired polymer articles.
Polymer Uses
The polyimide polymer produced as described above can be used for
several specific purposes. One important characteristic to consider
is the color of the polymer. Polyimide polymers usually absorb the
shorter wavelengths of light up to a specific wavelength, which can
be referred to as the 50% transmittance wavelength (50% T). Light
with wavelengths longer than the 50% transmittance wavelength are
generally not absorbed and pass through the polymer or are
reflected by the polymer. The 50% T is the wavelength at which 50%
of the electromagnetic radiation is transmitted by the polymer. The
polymer will tend to transmit almost all the electromagnetic
radiation above the 50% T, and the polymer will absorb almost all
the electromagnetic radiation below the 50% T, with a rapid
transition between transmittance and adsorption at about the 50% T
wavelength. If the 50% T can be shifted to a point below the
visible spectrum, the polymer will tend to be very clear, but if
the 50% T is in or above the visible spectrum, the polymer will be
colored.
Generally, the factors that increase the solubility of a polymer
also tend to push the 50% T lower, and thus tend to reduce the
color of a polymer. Therefore, the factors that tend to reduce
color in a polymer include flexible spacers, kinked linkages, bulky
substituents, and phenyl groups which are aligned in different
planes. The current invention provides a polyimide polymer with
very little color.
A polyimide polymer with low color is useful for several
applications. For example, if a polyimide is used as a cover in a
multi layer insulation blanket on a satellite, the absence of color
minimizes the amount of electromagnetic radiation that is absorbed.
This minimizes the heat absorbed when the polymer is exposed to
direct sunlight. Temperature variations for a satellite can be
large, and a clear polyimide polymer, especially one that is
resistant to AO degradation, provides an advantage.
Display panels need to be clear, so as not to affect the quality of
the displayed image. The current invention is useful for display
panels. In addition to optical clarity, a display panel should have
low permeability to water and oxygen, a low coefficient of thermal
expansion, and should be stable at higher temperatures. Thermal
stability at 200 degrees centigrade is desired, but stability at
250 degrees centigrade is preferred, and stability at 300 degrees
centigrade is more preferred. The surface of the current polymer
can be glassified by exposing the surface to an oxygen plasma. This
causes the OS groups to degrade to a glass type substance, which
tends to coat and protect the surface of the polymer. This glass
layer is thin enough that the layer bends and flexes without
breaking. The glassified layer protects the polymer, and if the
glassification layer is thick enough it could lower the
permeability of the resultant polymer to water and oxygen. After
the surface of the OS containing polymer is glassified, the
permeability to oxygen and water vapor tends to be reduced, so the
need for additional measures to lower this permeability are reduced
or eliminated. One example of an additional measure which could be
used to lower permeability is to thicken the glassification layer
by depositing silicon oxide on the surface of the film. The effects
of any traces of color in the polymer are minimized by supplying
the polymer in a thin film, such as 1 mil thick. Thicker films are
more durable, but they are also heavier and tend to have greater
color effects, with the opposite being true for thinner films.
Polyimide films tend to be very strong, so they can be used as
protective covers. For example, sheets of polyimide film can be
placed over solar panels to protect the panels from weather and
other sources of damage. For a solar panel to operate properly, it
has to absorb sunlight. Polyimide polymers with low color are
useful to protect solar panels, and other items where a view of the
protected object is desired.
EXAMPLES
Example 1
To a clean, dry, 5 liter (1) reactor equipped with an overhead
stirrer, thermometer, and rubber septa were added 428.26 grams (g)
APB, 81.88 g DBA, and 5591.08 g DMAc. The reactor was sealed and
purged with dry argon while the solution was stirred vigorously
with the overhead stirrer until the regents dissolved. To this
solution was added 898.76 g 6FDA, and the resultant slurry was
stirred for 24 hours, at which point 13.78 g phthalic anhydride
(PA) was added to the reaction vessel and allowed to react for 4
hours. 501.18 g pyridine and 646.84 g acetic anhydride were added
to this solution. The solution was stirred for 24 hours at room
temperature, 24 hours at 70.degree. C., then cooled to room
temperature and precipitated in deionized water. The recovered
polymer, which is referred to as SPC, was washed three times with
water, and dried overnight in a vacuum oven at 100.degree.
centigrade (C.) to yield 1374.81 g dry SPC (98% yield).
To a clean, dry, 5 L reactor equipped with an overhead stirrer,
thermometer, and rubber septa were added 900.00 g of SPC prepared
as described above and 5406.75 g dichloromethane (DCM). The reactor
was sealed and purged with dry argon while the solution was stirred
until homogeneous. 398.75 g aminopropylisobutyl polyhedral OS,
86.55 g Dicyclohexyl carbodiimide (DCC), and 1802.25 g DCM, and
891.00 g dimethylacetamide (DMAc) were added to this solution. The
reaction was allowed to proceed for 24 hours at room temperature.
The reactor was then cooled to 0.degree. C. for three hours during
which time the contents became heterogeneous. The contents of the
reactor were drained, and filtered to remove the precipitate. The
recovered polymer solution was precipitated into ethanol,
recovered, and rinsed three times with deionized water. The
OS-containing polymer was then dried in a vacuum oven at
110.degree. C. for 48 hours to yield 1148.30 g dry polymer (94%
yield).
Example 2
To a clean, dry, 5 L reactor equipped with an overhead stirrer,
thermometer, and rubber septa were added 1050.00 g of SPC prepared
in Example 1 and 4000.00 g tetrahydrofuran (THF). The reactor was
sealed and purged with dry argon while the solution was stirred
until homogeneous. 403.18 g aminopropylisobutyl polyhedral OS,
87.51 g DCC, and 2090.00 g THF were added to this solution. The
reaction was allowed to proceed for 24 hours at room temperature.
The reactor was then cooled to 0.degree. C. for three hours during
which time the contents become heterogeneous. The contents of the
reactor were drained, and filtered to remove the precipitate. The
recovered polymer solution was precipitated into deionized water,
recovered, and rinsed three times with deionized water. The
OS-containing polymer was then dried in a vacuum oven at 10.degree.
C. for 48 hours to yield 1207.90 g dry polymer (98% yield).
CONCLUSION
The process described is very efficient, including variations with
only one or even zero isolations of the polyimide polymer. This is
achieved by removing water as a vapor during either the reaction
where the polyimide polymer is formed or the reaction where the OS
group is attached to the polyimide polymer, or both. The entire
process is completed in only one or two pots, and each reaction
goes to a high rate of completion. If an isolation of the polymer
is performed, it is a simple precipitation and filtration
separation, which tends to be relatively efficient. There is no
need to use chromatography or other more difficult separation
methods. The more difficult separation methods limit the amount of
product which can be produced in a single batch, and the speed with
which a batch can be processed. The preferred monomers are
commercially available, and this results in a less expensive, more
efficient process with the possibility of large production
rates.
While the invention has been described with respect to a limited
number of embodiments, those skilled in the art, having benefit of
this disclosure, will appreciate that other embodiments can be
devised which do not depart from the scope of the invention as
disclosed here. Accordingly, the scope of the invention should be
limited only by the attached claims.
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